Conventionally, among displacement detection apparatuses used in the field of manufacturing semiconductors, etc., there is known what is called a linear encoder which has a solid scale having a graduation marked thereon and a detection unit for electrically detecting displacement of the solid scale along the linear movement direction thereof. Among such linear encoders, as a displacement detection apparatus with high accuracy and high resolution, there is known a hologram encoder, etc. for detecting displacement using interference of diffracted lights.
FIG. 1 shows the configuration of a conventional displacement detection apparatus 101. The displacement detection apparatus 101 detects displacement using interference of diffracted lights generated by a diffraction grating.
The displacement detection apparatus 101 generally includes a light emitting unit 102, an optical path control and objective unit 103, and an optical receiving unit 104, each of which has following parts and components.
The light emitting unit 102 has a semiconductor laser (LS), a converging lens (L1), and a polarization beam splitter (BS1).
The optical path control and objective unit 103 has reflecting mirrors (R1a, R1b), a reflection type diffraction grating (RG), converging lenses (L2a, L2b), λ/4 wave plates (WP1a, WP1b), and reflecting mirrors (R2a, R2b).
The optical receiving unit 104 has a semitransparent mirror (HM), polarization beam splitters (BS2, BS3), a λ/4 wave plate (WP2), and photodetectors (PD1 to PD4).
A light emitted from the semiconductor laser LS being a light source arranged in the light emitting unit 102 is converged by the converging lens L1, and thus converged light is polarized and split into a light LFa and a light LFb by the polarization beam splitter BS1. The light LFa has its optical path deflected by the reflecting mirror R1a, and thus deflected light LFa goes to the reflection type diffraction grating RG. On the other hand, the light LFb has its optical path deflected by the reflecting mirror R1b, and thus deflected light LFb goes to the reflection type diffraction grating RG. When the converged light from the converging lens L1 is polarized and split into the light LFa and the light LFb, the converged light is split into the P polarization component and the S polarization component.
Diffracted lights from the reflection type diffraction grating RG arranged in the objective unit (such as a linear scale) whose order of diffraction are of the same sign (of the same positive sign or of the same negative sign) and are higher than at least the first order are transmitted or pass through the converging lenses L2a, L2b to be converged, respectively. Then, thus converged lights have their polarization direction rotated by a right angle by the λ/4 wave plates WP1a, WP1b, respectively, which are arranged corresponding to the diffraction angle. Then, the lights passing through the λ/4 wave plates WP1a, WP1b are reflected by the reflecting mirrors R2a, R2b, respectively. Then, the reflected lights go backward along thus followed optical path to the polarization beam splitter BS1.
Since each of the lights coming to the polarization beam splitter BS1 has its polarization direction rotated by a right angle against the original direction thereof, the lights which pass through the polarization beam splitter BS1 are oriented along a direction toward the semitransparent mirror HM, which is different from that toward the semiconductor laser LS. Then, the lights which pass through the semitransparent mirror HM are split into two, one of which goes to the polarization beam splitter BS3, while the other of which goes to the polarization beam splitter BS2 after passing through the λ/4 wave plate WP2, respectively.
The polarization beam splitter BS3 is so arranged as to be rotated around the optical path being its rotation center by 45 degrees against the polarization direction of the coming light.
The light which comes to the polarization beam splitter BS2 is polarized and split into two which go to the photodetectors PD1, PD2, respectively, where intensity of the lights are converted into electrical signals. On the other hand, the light which comes to the polarization beam splitter BS3 is polarized and split into two which go to the photodetectors PD3, PD4, respectively, where the intensity of the lights are converted into electrical signals.
Next, the basic principle of the displacement detection apparatus 101 will be explained.
The light LFa and the light LFb of different polarization direction or of different polarization state which are split by the polarization beam splitter BS1 become the diffracted lights of the same sign when diffracted by the reflection type diffraction grating RG. Then, the diffracted lights have their polarization direction rotated by substantially a right angle when passing through the λ/4 wave plates WP1a, WP1b, respectively. Then, the resulting lights are reflected by the reflecting mirrors R2a, R2b, respectively, and are returned to the polarization beam splitter BS1 to be combined.
When the light LFa and the light LFb are combined, since they are split from the light emitted from the semiconductor laser LS and have the same polarization component, there is generated interference even though the polarization direction thereof are different from each other.
When the reflection type diffraction grating RG is caused to move in a direction such as an arrow A along which the grating is aligned relatively with other optical systems, the light LFa and the light LFb which are combined by the polarization beam splitter BS1 interfere with each other. Thus, intensity variation is generated in a pitch corresponding to the diffraction order for each polarization direction, the photodetectors PD1 to PD4 can detect the intensity variation as light intensity distributions whose phases are different from each other by separating the intensity variation attributable to the interference into a plurality of polarization components. That is, the movement or displacement of the reflection type diffraction grating RG can be detected by detecting the light intensity distributions with resolution of a pitch of the diffraction grating multiplied by one half of the reciprocal of the diffraction order and the reciprocal of the diffraction frequency. Also, the shape of the intensity variation obtained by the photodetectors PD1 to PD4 is extremely similar to that of a sinusoidal wave, higher resolution can be obtained by dividing a detected wave form in the interpolation manner.
In such a displacement detection apparatus using interference of diffracted lights, it becomes possible to realize resolution of the order of nm (nanometer) by using a reflection type diffraction grating made out of a hologram and dividing a sinusoidal wave generated therefrom.
However, in the manufacturing process of the conventional displacement detection apparatus, when assembling parts or units such as a light emitting unit, an optical receiving unit, and other optical parts, each of which is independently made from others, it is necessary to adjust those units or parts in respective manufacturing steps. So, there are raised following problems.
That is, firstly, in assembling parts of the apparatus, since accurate adjustment is required to remove or offset inequality of accuracy or characteristics of respective finished parts, complicated manufacturing steps have to be inevitably prepared. So, it is difficult to reduce the manufacturing cost, and to reduce assembling machines in size since wide working space is required for adjusting, clamping, and fixing respective parts. Secondly, since adhesive is required when fixing respective parts after accurate adjustment, and the state of adhesion is prone to be affected by environmental variation etc., assembling shear attributable to such environmental variation and variation per hour may be undesirably raised.